Teachers Guide
Resonance Tube demonstration This experiment may be demonstrated by the teacher to the whole class or, if there is enough equipment, students may work in pairs to complete the experiment themselves. Before the students attempt the experiment or view the demonstration they should be directed to read the 'Background to Sound Waves and Resonance' section on this site. Equipment needed: * Tuning forks of differing frequencies. * Rubber block to strike the tuning forks on. * Large glass or plastic cylinder, around 1000ml is suitable. * Long hollow tube with a length long enough so at least one resonance can be found for each tuning fork. * Metre ruler. * Clamp to hold the metre ruler beside the tube. See Figure 1 for an example of the experimental set up. Assuming the classroom contains approximately dry air at 20°C, the speed of sound should be around 343 meters per second (m/s). For a tuning fork of frequency f , 343={\lambda}{f} Therefore, knowing the values for the frequencies of the tuning forks, it is possible to calculate the minimum length tube that will be needed to observe resonance. The hollow tube must have: Length>\dfrac{343}{2f} This is because, for resonance to be heard, the open end of the tube must be an anti-node as the closed end is a node. If the length of the tube is just over half a wavelength of the sound wave, the wave reflected back will travel a whole wavelength before you hear the resonance (Figure 2). Use this to source a suitably long hollow tube for use with the tuning forks available. The equation used for calculating the speed of sound knowing the length of the tube at resonance is: v=\frac{4f}{n}(L+0.3D) Where f is the frequency of the tuning fork, n is an odd integer corresponding to the specific harmonic being measured, L is the length of the tube out of the water and D is the diameter of the tube. If students are attempting the experiment in pairs then they should be directed to read the relevant 'Measuring resonance and the speed of sound: Resonance Tube' page on the wiki. This page will still be helpful for the students understanding even if the teacher is demonstrating the experiment. If a teacher is demonstrating then the following method should be followed: # Measure and record the diameter of the tube. # Fill the beaker almost to the top with water and place the hollow tube inside. # Ring the tuning fork on a rubber block. # Hold the ringing fork above the tube. # Move both the tuning fork and tube up until the volume of the tuning fork is increased. # Measure the height of the tube above the water at the point at which the volume is greatest. # Once one resonant point has been measured, continue raising the tube to look for/measure more resonance points. The tube should remain at least partially submerged throughout the resonance testing. # Repeat the measurements two more times and calculate an average to get more accurate results. # Repeat the method for the other tuning forks of different frequencies. # Use the values obtained and the equation for the speed of sound in a resonance tube to calculate the speed of sound. Sample Results For these sample results, a smaller hollow tube of diameter (2.1 \pm 0.1)cm with a length of 35 cm was used for shorter harmonics and a larger hollow tube of diameter (4.4 \pm 0.1)cm with a length of 80 cm was used for longer harmonics. Results obtained are shown below. By averaging the resonance length for each harmonic and tuning fork, then using the equation given above to calculate the speed of sound for each harmonic and averaging all values, the speed of sound is measured to be 342m/s , which is <1% error from the accepted value at room temperature and atmospheric pressure. Raw measurements: Here are the results in graphical form: